The present invention relates to an improvement in a device for injection of liquid loads into an industrial system of gas chromatography, involving large diameter columns.
As already set forth in French Pat. No. 2,227,890, assigned to the same assignee of the instant application, the injection of the liquid load, into a gas chromatography separation device, constitutes an important element in the obtaining of a good separation in the chromatographic column. As a matter of fact, the highest efficiency is obtained when the curve of fluctuation of the concentration of the injected product, as a function of the time, exhibits a rectangular form for the injection time and the time of a cycle, as can be seen in FIG. 1 in the attached drawing. In this figure, the times are plotted in the abscissa and the concentrations of the product injected into the vector gas is plotted in the ordinate.
The embodiment of an injection of this type, prevents the passage into the column, before or after the injection, of trails of the mixture to be separated, which would upset the equilibrium inside the column between the phases present, and would lead to a poor resolution of the mixture to be separated.
In classic fashion, the liquid is introduced into a vaporizer and the gas which is produced is transported into the column by the motive force of the vector gas. The start of the injection is controlled by the opening of a valve placed in the piping that feeds the liquid to the vaporizer, and the end of the injection by the closing of this same valve. This technique leads to injections of non-rectangular form, with a trail due to the vaporization of small quantities of the product to be separated remaining in the vaporizer after the injection is stopped. Thus, there is described in the aforesaid French patent, a method and device for injection of liquid into a gas chromatography column with a vaporizer continuously swept by a vector gas, the liquid being brought to the vaporizer by a pipe equiped with a valve which is opened and then closed to permit the injection at a given rate of flow, of the desired quantity of liquid to be injected, a fast purge being made at the end of the injection in the pipe between the valve and the vaporizer, as well as a continuous purge at a very low rate of flow between the injection periods.
This device, used for columns 125 mm in diameter, is represented by the schematic in FIG. 2; the preheated vector gas is introduced at 8 and continuously sweeps the vaporizer-exchanger 6 before entering the chromatographic column 9 where it is distributed through a fritted plate 10; the liquid product to be processed is introduced at 3.
During the injection, valve 3 is opened and fast-purge valve 2 closed; the liquid is then vaporized very rapidly in the presence of the vector gas in exchanger 6. To improve the form of the injection "crenellations" valve 2 is opened for a few seconds, immediately after the end of the injection in order to sweep the portion of line comprised between vaporizer 6 and valve 3 and then filled with product to be processed. The manual microvalve 1 allows continuous passage of a very small flow in order to palliate a slight leakage of remote-controlled injection valve 3.
It has been found that this device has the following drawbacks:
constant overheating of the vector gas between the periods of injection of the mixture to be vaporized and separated,
"Trailing" of injection crenellations inducing less separation in the column, particularly for large-dimensioned vaporizers (column with a diameter of 400 mm and more),
Risk of overheating and cracking of small quantities of products to be separated--which are, generally speaking, thermally fragile products--inducing a clogging of the fritted input piece in the column, and a gradual coking of the sides of the vaporizer; these cases of stoppage calling for long cleaning periods that seriously affect the economy of the method.